Run bash completion command for given command

Question

I am trying to create a bash completion for a custom function that I am creating:

gco() {
    if [ -z "$1" ]; then
        branch="$(git branch --all | fzf | tr -d '[:space:]')"
    else
        branch="$1"
    fi
    git checkout "$branch"
}

I want to be able to delegate the auto completion of my gco function to the auto completion for git checkout, so I tried adding this:

function _gco() {
    local curr_arg;
    curr_arg=${COMP_WORDS[COMP_CWORD]}
    COMPREPLY=( $(compgen -W "$(git checkout)" -- $curr_arg ) );

}
complete -F _gco gco

However, when I type in gco and then press tab, I get the following output:

$ gco 
1           different     into                respectively.    yours)
38          diverged,     merge               the
and         each,         origin/mybranch     to
branch      git           pull                (use
commits     have          remote              Your

I think the issue is with this line:

COMPREPLY=( $(compgen -W "$(git checkout)" -- $curr_arg ) );

I don't know what the auto completion script is for git checkout, but I want to know if there is a way I can run the auto completion script for any arbitrary command without actually running said arbitrary command:

_run_auto_completion_script_for_command "git checkout"

Is there a way I can do this?